Ink jet printer

ABSTRACT

An ink jet printer that produces a multi-value output of a multi-tone image. The printer includes a main scan driving unit that drives print head with two nozzle arrays in a main scanning direction relative to a printing medium. Each nozzle array has a preset number of nozzles arranged in a fixed nozzle interval. A sub-scan driving unit drives the printing medium in a sub-scanning direction. A driving unit controller controls the main scan driving unit and the sub-scan driving unit to locate the print head at the determined positions. The print head driving unit supplies electric power for the print head based on print image data which includes multi-value tone information which is stored in the data storage unit. The binary output is based on the jetting or nonjetting of ink and the positional control by the driving unit controller causes the print head driving unit to superpose dots on the dots that have already been formed so as to carry out multi-value outputs.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ink jet printer that produces amulti-value output of a multi-tone image, such as a photographic image.

2. Discussion of the Background

An ink jet printer generally spouts a specific ink from ink jet nozzleson a printing medium to form small dots to execute printing. In aconcrete procedure, the following procedure is repeated to execute theprinting: executing dot printing while driving in a main scanningdirection a nozzle array having a plurality of nozzles arranged in asub-scanning direction; feeding a paper sheet at a predetermined pitchin the sub-scanning direction; and executing the dot printing againwhile driving the nozzle array in the main scanning direction.

Print outputs made by the ink jet printer are not restricted to theconventional print of letters, but it is required to print a multi-toneimage, such as a photographic image, with a high quality. The ink jetprinter has been improved to satisfy such a requirement, therebyattaining higher resolution and enabling the printing with the finerdots. According to a generally used method of producing a multi-valueoutput of a multi-tone image, driving frequency of the ink jet nozzlesin the main scanning direction is made approximately twice an ordinaryfrequency while the driving distance is minutely regulated so as tochange the pixel density.

FIG. 1 shows the concept of a conventional multi-value output technique.This example shows dot formation by three-value outputs, based on printimage data including four-value tone information. The four-value toneinformation requires at least 2-bit, and in the example of FIG. 1(a),8-bit (b7-b0) raster byte data constitutes print image data of fourpixels. Two-bit combinations for expressing each pixel are (b7,b6),(b5,b4), (b3,b2), and (b1,b0) as shown in FIG. 1(b). The 2-bitrepresenting the tone of one pixel expresses three value outputs byassigning the value ‘00’ to no output of dots, ‘01’ and ‘10’ to outputsof one dot, and ‘11’ to an output of adjoining two dots.

In the above conventional ink jet printer, it is required to drive theink jet nozzles at a driving frequency that is twice an ordinaryfrequency in order to carry out the multi-value outputs if the mainscanning speed is fixed. This requires the higher-speed head drivingmechanism, which undesirably increases the required cost. It may bepossible to maintain the driving frequency of the head while halving themain scanning speed only in the case of the multi-value outputs. This,however, lowers the throughput of printing to one half and increases thecontrol conditions on the main scanning speed.

Some conventional ink jet printers adopt a printing scheme offixed-pitch sub-scans, in order to attain high-quality printing. Thisprinting scheme controls the pitch of sheet feed in the sub-scanningdirection to be a constant value so that adjoining lines in thesub-scanning direction are formed by the dots spouted from different inkjet nozzles (see U.S. Pat. No. 4,198,642). When sheet feed errors areaccumulated under the minute sheet feeding control requirement, theabove multi-value outputs tend to cause banding.

The nozzle pitch has been narrowed to enhance the printing resolution,but there is a manufacture limit to simply narrow the nozzle pitch.Accordingly, print heads as shown in FIG. 2 are commercially available,in which plural columns (two columns in this example) of nozzle arraysare arranged apart from each other in the sub-scanning direction toapparently narrow the nozzle pitch (k pitch in the illustrated example).In such conventional print heads, banding due to a positionaldisplacement of the nozzles easily occurs if the head is inclined. Thewider distance between the adjoining columns of the nozzle arrays makesthe banding (that is, the streak-like pattern formed along thesub-scanning direction) more conspicuous.

In the conventional multi-value output technique, dots are formedconsecutively in the transverse direction in the case of three-valueoutputs. The dot shape accordingly tends to be long from side to side asshown in FIG. 1(b). This lowers the image quality due to granularitydeterioration, and requires more accurate sheet feed control because thedots do not extend in the vertical direction.

SUMMARY OF THE INVENTION

An object of the present invention is thus to provide an ink jet printerthat effectively alleviates the occurrence of banding without requiringcomplicated control and that ensures high-quality multi-value outputs.

In order to attain at least part of the above object, an ink jet printeraccording to the present invention comprises: a print head having aplurality of nozzles; a main scan driving unit that drives the printhead in a predetermined main scanning direction relative to a printingmedium; a sub-scan driving unit that drives and feeds the printingmedium in a sub-scanning direction, which is perpendicular to the mainscanning direction; a driving unit controller that controls the mainscan driving unit and the sub-scan driving unit to position the printhead at predetermined locations; a data storage unit that stores printimage data including multi-value tone information; and a printhead-driving unit that supplies electric power to the print head to jetink onto the printing medium based on the print image data stored in thedata storage unit; the print head including a plurality of nozzlegroups, each nozzle group forming dots of substantially identical color,the print head being driven to enable each nozzle group to record allpixels in an effective recording area on the printing medium; whereinthe print head-driving unit has a multi-value output mode in which theprint head is driven so that the print head can put a plurality of dotshaving the substantially identical color one upon another at anidentical position using the plurality of nozzle groups, to thereby formmulti-value dots representing multi-levels.

By superposing a plurality of dots having a substantially identicalcolor one upon another, three or more tone levels can be expressed byone dot.

It is preferable that the print head driving unit puts the plurality ofdots having the substantially identical color one upon another so thatthe multi-value dots is substantially circular. This arrangementeffectively prevents the occurrence of banding.

It is further preferable that the plurality of dots having thesubstantially identical color include a first density dot having arelatively low density and a second density dot having a relatively highdensity; wherein the multi-levels include a first tone level attained bythe first density dot, a second tone level attained by the seconddensity level, and a third tone level attained by superposing the firstand second density dots; and wherein the plurality of nozzle groupsinclude at least one nozzle group for each of the first and seconddensity dots, respectively. This arrangement effects to record dotshaving multi-tone levels with a plurality of different density inks.

It is also preferable that the plurality of nozzle groups include atleast two nozzle groups for at least one of the first density dot andthe second density dot, the at least two nozzle groups being able torecord all the pixels in the effective recording area; and wherein themulti-levels further include a tone level at which the at least nozzlegroups are used to superpose a plurality of identical density dots oneupon another. Alternatively, the plurality of nozzle groups may includeat least two nozzle groups for each of the first density dot and thesecond density dot, the at least two nozzle groups being able to recordall the pixels in the effective recording area; and wherein themulti-levels further include a fourth tone level at which a plurality ofthe first density dots are laid one upon another and a fifth tone levelat which a plurality of the second density dots are laid one uponanother.

It is also preferable that the data storage unit includes a plurality ofdata blocks for an identical ink, each of the plurality of data blocksstoring one bit of pixel information of print image data; and whereinthe plurality of data blocks are related to the plurality of nozzlegroups so that 1-bit print image data in each data block is used as datafor the related nozzle group. The supply of 1-bit print image data fromeach data block to the nozzles in the related nozzle group effectivelycontrols the jetting or non-jetting of nozzles in the nozzle group.

It is preferable that each of the plurality of nozzle groups includes Nnozzles (N being a positive integer) arranged at a nozzle interval k (kbeing a positive integer) in the sub-scanning direction; and whereinwhen the number of used nozzles in the sub-scanning direction in eachnozzle group used for printing is equal to n (n being a positive integerof not greater than N), k and n are prime to each other.

The plurality of nozzle groups may include an even nozzle array and anodd nozzle array, each having N nozzles (N being a positive integer)arranged at a nozzle interval 2k (k being a positive integer) in thesub-scanning direction, and the even and odd nozzle arrays are apartfrom each other by a predetermined distance in the main scanningdirection; and wherein when the number of used nozzles in thesub-scanning direction in each of the even and odd nozzle arrays usedfor printing is equal to n (n being a positive integer of not greaterthan N), 2k an n are prime to each other.

If any one of the above relationships between k and n is satisfied, thedriving unit controller can feed the medium in a medium-feed operationmode in which a feed amount of the sub-scan driving unit is fixed to ndots.

Alternatively, the driving unit controller may use a combination of aplurality of different values for feed amounts of a plurality ofsub-scans. A variety of scanning schemes capable of recording all thepixels with dots are applicable irrespective of whether the nozzleinterval and the number of used nozzles are prime to each other or not.

It is preferable that the print head carries out a plurality ofink-droplet jetting operations for the plurality of dots of thesubstantially identical color, the plurality of operations being carriedout in different main scans, respectively. This arrangement makes theinterval of the operations for jetting ink droplets to be a period ofone main scan or more, thereby preventing a blot of the ink droplet.

A computer readable recording medium according to the present inventionis on storing a computer program used in a computer that comprises aprint head having a plurality of nozzle groups and a data storage unit,each nozzle group forming dots of a substantially identical color, thedata storage unit storing print image data including multi-value toneinformation, the computer program being used for forming dots on aprinting medium with the print head, the computer program causing thecomputer to implement: a main scan driving function for driving theprint head in a predetermined main scanning direction relative to theprinting medium; a sub-scan driving function for driving and feeding theprinting medium in a sub-scanning direction that is perpendicular to themain scanning direction; a driving unit control function for controllingthe main scan driving function and the sub-scan driving function tolocate the print head at predetermined positions; and a print headdriving function for controlling spout of ink droplets on the printingmedium based on the print image data stored in the data storage unit,wherein the print head driving function has a multi-value output mode inwhich a plurality of dots having the substantially identical color arelaid one upon another at an identical position by the plurality ofnozzle groups, to thereby form multi-value dots representingmulti-levels. When the computer program is executed by the computer,three or more tone levels can be expressed by one dot in a similarmanner as in the above ink jet printer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) show the concept of a conventional multi-valueoutput technique;

FIG. 2 shows a print head having two nozzle arrays of even and oddarrays in order to effect a narrower pitch;

FIG. 3 is a block diagram schematically illustrating the structure of animage processing system applied to the present invention;

FIG. 4 shows the internal structure of the computer 90 and itsconnection with a network;

FIG. 5 schematically illustrates the structure of a color printer 22 asan example of the image output apparatus 20;

FIG. 6 shows the structure of a print head 28;

FIG. 7 shows the principle of an ink jetting operation;

FIGS. 8(A) and 8(B) show an arrangement of ink jet nozzles on inkdischarge heads 61-64;

FIG. 9 shows the structure of an ink jet printer in a first embodimentaccording to the present invention;

FIG. 10 shows an example of raster blocks in a data storage unit;

FIG. 11 shows the concept of a multi-value output technique of theembodiment;

FIG. 12(a) shows a process of forming an initial dot according to themulti-value output technique of the embodiment, and FIG. 12(b) shows aprocess of superposing ink upon the existing dot;

FIG. 13 shows the dot forming positions in a plurality of scanningpasses;

FIG. 14 shows the structure of another ink jet printer in a secondembodiment according to the present invention;

FIG. 15 shows the dot forming positions in a plurality of scanningpasses of a dark color nozzle array for jetting a high density ink;

FIG. 16 shows the dot forming positions in a plurality of scanningpasses of a light color nozzle array for spouting a low density ink;

FIG. 17 shows the sequence of forming dark color dots and light colordots;

FIG. 18 shows the relationship between the tone value, the ink density,and the resulting dot;

FIGS. 19(A) and 19(B) show the fundamental conditions of generalscanning schemes when the number of scan repeats s is equal to 1;

FIGS. 20(A) and 20(B) show the fundamental conditions of generalscanning schemes when the number of scan repeats s is not less than 2;

FIG. 21 shows a first scanning scheme using a plurality of differentsub-scan feed amounts;

FIGS. 22(A) and 22(B) shows scanning parameters and raster numbers ofeffective raster lines recorded by the respective nozzles in the firstscanning scheme;

FIG. 23 shows the nozzle numbers for recording the effective rasterlines in the first scanning scheme;

FIGS. 24(A) and 24(B) show scanning parameters and raster numbers ofeffective raster lines recorded by the respective nozzles in a secondscanning scheme using a plurality of different sub-scan feed amounts;

FIG. 25 shows nozzle numbers for recording effective raster lines in thesecond scanning scheme;

FIG. 26 shows a scanning scheme when an offset G of the sub-scan feedamount L is a constant value;

FIG. 27 shows combinations of a nozzle pitch k with desirable offsets Gof the sub-scan feed amount;

FIGS. 28(A) and 28(B) show scanning parameters and raster numbers ofeffective raster lines recorded by the respective nozzles in a thirdscanning scheme using a plurality of different sub-scan feed amounts;

FIG. 29 shows nozzle numbers for recording effective raster lines in thethird scanning scheme;

FIG. 30 shows scanning parameters in a fourth scanning scheme using aplurality of different sub-scan feed amounts;

FIG. 31 shows raster numbers of effective raster lines recorded by therespective nozzles in the fourth scanning scheme; and

FIG. 32 shows nozzle numbers for recording effective raster lines in thefourth scanning scheme.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A. Structure of Apparatus

FIG. 3 is a block diagram illustrating the structure of a color imageprocessing system embodying the present invention. The color imageprocessing system includes a scanner 18, a personal computer 90, and acolor printer 22. The personal computer 90 includes a color display 21.The scanner 18 captures color image data of a color original, andsupplies the original color image data ORG, including R, G, and Bcomponents, to the computer 90.

The computer 90 is provided therein with CPU, RAM, and ROM (not shown),and an applications program 95 runs under a specific operating system. Avideo driver 91 and a printer driver 96 are incorporated in theoperating system, and final color image data FNL of the applicationsprogram 95 are output through these drivers. The applications program 95used to, for example, retouch an image, reads an image from the scanner,execute a prescribed processing, and displays the image on the CRTdisplay 93 through the video driver 91. When the applications program 95outputs a printing instruction, the printer driver 96 receives imageinformation from the applications program 95 and converts the inputimage information to printing signals for the printer 22. (The printingsignals are binarized signals for the respective colors of C, M, Y, andK.) In the example of FIG. 1, the printer driver 96 includes: arasterizer 97 for converting the color image data processed by theapplications program 95 to dot-based image data; a color correctionmodule 98 for executing color correction on the dot-based image dataaccording to the ink colors of C, M, and Y used by the printer 22 andthe colorimetric characteristics of the printer 22; a color correctiontable CT referred to by the color correction module 98; a halftonemodule 99 for generating halftone image data, which represents imagedensity in a particular area by on/off of ink in each dot, from thecolor-corrected image data; and mode selection writing module 110 forwriting mode selection information, which will be described later, intoa memory in the color printer 22.

FIG. 4 is a block diagram illustrating the internal structure of thecomputer 90. The computer 90 includes a CPU 81, which executes a varietyof arithmetic and logic operations according to computer programs inorder to control operations related to image processing, and thefollowing units mutually connected to one another via a bus 80. A ROM 82stores computer programs and data required for execution of the varietyof arithmetic and logic operations by the CPU 81. A RAM 83 is a memory,which temporarily stores various computer programs and data required forexecution of the variety of arithmetic and logic operations by the CPU81. An input interface 84 receives input signals from the scanner 18 anda keyboard 74, whereas an output interface 85 sends output data to theprinter 22. A CRT controller (CRTC) 86 controls signal outputs to a CRT21 that can display color images. A disk drive controller (DDC) 87controls transmission of data from and to a hard disk 76, a flexibledrive 75, and a CD-ROM drive (not shown). The hard disk 76 stores avariety of computer programs that are loaded into the RAM 83 andexecuted, as well as other computer programs that are supplied in theform of device drivers. A serial input-output interface (SIO) 88 is alsoconnected to the bus 80. The SIO 88 is connected to a modem 78 andfurther to a public telephone network PNT via the modem 48. The computer90 is connected with an external network via the SIO 88 and the modem78, and can access a specific server SV in order to download thecomputer programs for image processing into the hard disk 76. Thecomputer 90 may alternatively execute the required programs which havebeen loaded from a flexible disk FD or a CD-ROM.

FIG. 5 schematically illustrates the structure of the printer 22. Asshown in the drawing, the printer 22 has a mechanism for feeding a sheetof paper P by means of a sheet feed motor 23, a mechanism forreciprocating a carriage 31 along the axis of a platen 26 by means of acarriage motor 24, a mechanism for driving a print head 28 mounted onthe carriage 31 to control discharge of ink and formation of dots, and acontrol circuit 40 for transmitting signals to and from the sheet feedmotor 23, the carriage motor 24, the print head 28, and a control panel32.

A black ink cartridge 71 and a color ink cartridge 72 for storing pluralcolor inks can be mounted on the carriage 31 of the printer 22. Pluralink discharge heads 61 through 64 are formed on the print head 28 thatis disposed in the lower portion of the carriage 31, and ink supplyconduits 65 (see FIG. 6) are formed in the bottom portion of thecarriage 31 for leading supplies of ink from ink tanks to the respectiveink discharge heads 61 through 64. When the black ink cartridge 71 andthe color ink cartridge 72 are attached downward to the carriage 31, theink supply conduits 65 are inserted into connection apertures (notshown) formed in the respective cartridges. This enables supplies of inkto be fed from the respective ink cartridges to the ink discharge heads61 through 64.

The following briefly describes the mechanism of discharging ink. Whenthe ink cartridges 71 and 72 are attached to the carriage 31, inks inthe ink cartridges 71 and 72 are sucked out through the ink supplyconduits 65 by capillarity and are led to the ink discharge heads 61through 64 formed in the print head 28 arranged in the lower portion ofthe carriage 31 as shown in FIG. 6. When the ink cartridges 71 and 72are attached to the carriage 31, a pump works to suck first supplies ofink into the respective ink discharge heads 61 through 64. In thisembodiment, the structures of the pump for suction and a cap forcovering the print head 28 during the suction are not illustrated nordescribed specifically.

An array of thirty-two nozzles 200 is formed in each of the inkdischarge heads 61 through 64 as shown in FIG. 6. A piezoelectricelement PE, which is one of electrically distorting elements and has anexcellent response, is provided for each nozzle 200. FIG. 7 illustratesa configuration of the piezoelectric element PE and the nozzle 200. Thepiezoelectric element PE is disposed at a position that comes intocontact with an ink conduit 80 for leading ink to the nozzle 200. As isknown, the piezoelectric element PE has a crystal structure that issubjected to a mechanical stress due to application of a voltage andthereby carries out extremely high-speed conversion of electrical energyto mechanical energy. In this embodiment, application of a voltagebetween electrodes on either ends of the piezoelectric element PE for apredetermined time period causes the piezoelectric element PE to extendfor the predetermined time period and deform one side wall of the inkconduit 80 as shown in the lower part of FIG. 7. The volume of the inkconduit 80 is reduced with an extension of the piezoelectric element PE,and a certain amount of ink corresponding to the reduced volume issprayed as ink particles Ip from the ends of the nozzle 200 at a highspeed. The ink particles Ip soak into the sheet of paper P set on theplaten 26, so as to reproduce a print.

In the printer 22 of the embodiment having the hardware structurediscussed above, the sheet feed motor 23 rotates the platen 26 and theother related rollers to feed the printing paper P. The carriage motor24 drives and reciprocates the carriage 31, simultaneously withactuation of the piezoelectric elements PE on the respective inkdischarge heads 61 through 64 of the print head 28. The printer 22accordingly sprays the respective color inks and forms a multi-colorimage on the printing paper P. Concrete arrangements of the nozzles inthe respective ink discharge heads 61 through 64 will be discussedlater.

The mechanism for feeding the printing paper P includes a gear train(not shown) for transmitting rotations of the sheet feed motor 23 to theplaten 26 as well as a sheet feed roller (not shown). The mechanism forreciprocating the carriage 31 includes a sliding shaft 34 arranged inparallel with the axis of the platen 26 for slidably supporting thecarriage 31, a pulley 38, an endless drive belt 36 spanned between thecarriage motor 24 and the pulley 38, and a position sensor 39 fordetecting the position of the origin of the carriage 31.

The control circuit 40 includes a CPU (not shown), main memories havinga ROM and a RAM (not shown), and a programmable ROM (PROM) 42, which isa rewritable non-volatile memory. The PROM 42 stores dot recording modeinformation including parameters with respect to a plurality of dotrecording modes. The “dot recording mode” denotes a scanning schemedefined by the number of actually used nozzles N, the sub-scan feedamount L, and others. In the specification hereof, the terms “scanningscheme” and “recording mode” have substantially the same meanings.Concrete examples of the dot recording modes and their relatedparameters will be described later. Mode selection information is alsostored in the PROM 42 to select a desired mode among the plurality ofdot recording modes. For example, when the PROM 42 can store sixteenpieces of dot recording mode information, the mode selection informationconsists of four-bit data.

The dot recording mode information is read by the printer driver 96 fromthe PROM 42 when the printer driver 96 (FIG. 3) is installed at thestartup of the computer 90. In more concrete terms, the printer driver96 reads the dot recording mode information corresponding to a desireddot recording mode specified by the mode selection information from thePROM 42. The processes in the rasterizer 97 and the halftone module 99as well as the main scans and sub-scans are carried out according to thedot recording mode information.

The PROM 42 may be any rewritable non-volatile memory and is, forexample, an EEPROM or a flash memory. The dot recording mode informationmay be stored in a non-rewritable ROM, while it is preferable that themode selection information is stored in the rewritable non-volatilememory. Plural sets of dot recording mode information may be stored in astorage device other than the PROM 42 or alternatively in the printerdriver 96.

FIGS. 8(A) and 8(B) show an arrangement of ink jet nozzles in the inkdischarge heads 61 through 64. The first head 61 has a nozzle array forjetting black ink. Similarly the second through the fourth heads 62through 64 respectively have nozzle arrays for jetting respective inkswhich are different in color or density. These four nozzle arrays haveidentical positions in the sub-scanning direction.

Each of the four nozzle arrays includes thirty-two nozzles 200 arrangedin a zigzag manner with a constant nozzle pitch k in the sub-scanningdirection. The thirty-two nozzle 200 included in each nozzle array maybe arranged in alignment, instead of in the zigzag manner. The zigzagarrangement as shown in FIG. 8(A), however, has the advantage of beingable to set a smaller nozzle pitch k in the manufacturing process.

FIG. 8(B) shows an arrangement of a plurality of dots formed by onenozzle array. In this embodiment, driving signals are supplied to thepiezoelectric elements PE (FIG. 7) of the respective nozzles in order tocause a plurality of dots formed by one nozzle array to be arrangedsubstantially in alignment in the sub-scanning direction, regardless ofthe arrangement of the ink nozzles; that is, whether the nozzles arearranged in zigzag or in alignment. By way of example, it is assumedthat the nozzles are arranged in zigzag as shown in FIG. 8(A) and thatthe head 61 is scanned rightward in the drawing to form dots. In thiscase, a group of preceding nozzles 100, 102, . . . receive drivingsignals at an earlier timing by d/v [second] than a group of followingnozzles 101, 103 . . . . Here, d [inch] denotes a pitch between the twonozzle groups in the head 61 (See FIG. 8(A)), and v [inch/second]denotes the scanning speed of the head 61. A plurality of dots formed byone nozzle array are accordingly arranged in alignment in thesub-scanning direction. As described later, all of thirty-two nozzlesprovided in each of the heads 61 through 64 are not always used, butonly part of the nozzles may be used according to the scanning scheme.

The nozzle array in each ink jet head shown in FIG. 8(A) corresponds tothe dot forming element array of the present invention. The feedingmechanism of the carriage 31 including the carriage motor 24 shown inFIG. 5 corresponds to the main scan driving unit, and the feedingmechanism of the paper including the sheet feed motor 23 corresponds tothe sub-scan driving unit. Moreover, a circuit including thepiezoelectric element PE of each nozzle corresponds to the head drivingof the present invention. The control circuit 40 and the printer driver96 (FIG. 3) correspond to the control unit of the present invention.

B. First Embodiment

FIG. 9 is a functional block diagram of an ink jet printer 20 in a firstembodiment according to the present invention. The ink jet printer 20includes a print head 2, a main scan driving unit 3, a sub-scan drivingunit 4, a driving unit controller 5, a data storage unit 6, and a printhead driving unit 7. The print head 2 in FIG. 9 corresponds to the printhead 28 in FIG. 5, whereas the main scan driving unit 3, the sub-scandriving unit 4, and the print head driving unit 7 respectivelycorrespond to the carriage motor 24, the sheet feed motor 23, and thepiezoelectric element PE of FIG. 6. The driving unit controller 5 andthe data storage unit 6 correspond to the control circuit 40 in FIG. 5.

Like the example shown in FIG. 2, the print head 2 includes an evennozzle array 2 a and an odd nozzle array 2 b, which have the nozzleinterval 2k (where k is a positive integer) and the number of usednozzles n (seven nozzles are used where N=8 in the example shown in FIG.2) and are arranged at a predetermined interval in the main scanningdirection. When the sub-scan feed amount is a constant value, the nozzleinternal 2k and the number of used nozzles n are prime to each other.

The main scan driving unit 3 drives the print head 2 in a predeterminedmain scanning direction (the transverse direction in the drawing of FIG.9) relative to a printing medium S, such as a sheet of printing paper.The sub-scan driving unit 4 drives and feeds the printing medium S in asub-scanning direction (the vertical direction in the drawing of FIG.9), which is perpendicular to the main scanning direction.

The driving unit controller 5 regulates the driving amounts and thedriving timings of the main scan driving unit 3 and the sub-scan drivingunit 4, so as to shift the print head 2 in the main scanning directionto predetermined positions. The driving unit controller 5 implements amedium feeding operation mode, in which the feed amount of the printingmedium by the sub-scan driving unit 4 is a constant value of n dots,that is, the printing scheme using the fixed-pitch sub-scans describedabove. An example using non-constant sub-scan feed amounts will bedescrived later.

The data storage unit 6 has a memory, in which print image dataincluding multi-value tone information is stored. The memory has twodata block areas, that is, a raster block 0 and a raster block 1 asshown in FIG. 10. The respective raster blocks 0, 1 have 4-value toneinformation as the 2-bit combinations for each dot at an identicalposition. The dot formation data to be output to the even nozzle array 2a is stored in the raster block 0, whereas the dot formation data to beoutput to the odd nozzle array 2 b is stored in the raster block 1. Likethe prior art structure, the ink jet printer 1 of this embodimentexpresses three values by the 2-bit information at the correspondingpositions in the raster blocks 0, 1.

The print head driving unit 7 supplies electric power to the print head2, based on the print image data stored in the data storage unit 6,thereby jetting ink from desired nozzles in the even nozzle array 2 aand the odd nozzle array 2 b onto the printing medium S.

As shown in FIG. 11, the multi-value outputs of the ink jet printer 1 ofthe embodiment include no output of dots when the 2-bit datarepresenting the tone of each dot is equal to ‘00’, and one-dot outputby the standard sub-scan control if the 2-bit data is equal to ‘01’ or‘10’. If the 2-bit data is equal to ‘11’, the driving unit controller 5regulates the position of the print head 2 and spouts an ink droplet tooverlay a dot on an existing dot, thereby effecting the 3-value output.The dot formed by the 3-value output in this embodiment has a greaterdiameter than the dot by the 2-value output and a nearly complete roundshape.

The following describes the details of the 3-value output technique inthis embodiment with FIG. 12. As described previously, no ink jettingfrom a nozzle results in ‘dot-less state’, and ink jetting results in‘dot-formed state’. In the ‘dot-formed state’, ink deposited on theprinting medium S is gradually soaked into the printing medium S (seeFIG. 12(a)). When an ink droplet is deposited at the position where adot has already been formed, the newly deposited ink is soaked aroundthe previously deposited ink to form a larger dot (see FIG. 12(b)). Thisensures dot formation by the 3-value output.

An example of the multi-value outputs in this embodiment is describedwith the drawing of FIG. 13. FIG. 13 shows the dot forming positions ina plurality of scanning passes. In this example, while printing iscarried out according to the technique of fixed-pitch sub-scans, thedriving unit controller 5 controls to locate the even nozzle array 2 aand the odd nozzle array 2 b at predetermined identical positions. Inthe drawing of FIG. 13, the symbol O denotes dots formed by the evennozzle array, and the symbol □ denotes dots formed by the odd nozzlearray 2 b.

In the example of FIG. 13, the nozzle #8 of the even nozzle array 2 a inthe third main scan pass is located at the same dot forming position asthat of the nozzle #1 of the odd nozzle array 2 b in the seventh mainscan pass. Predetermined dots are then formed based on the 2-bitmulti-value tone data stored in the raster blocks 0, 1.

As described above, the multi-value output of this embodiment has thesame main scanning speed and head frequency as those in the normaloperation. Unlike the prior art, this does not increase the cost of thehead driving mechanism nor complicates the process of controlling themain scanning speed. The decrease in throughput is substantiallyequivalent to that when the main scanning speed is halved in the priorart. The dot shapes by the 3-value output in this embodiment isbasically a complete round, thereby reproducing high quality resultingimages.

In this embodiment, the dot by the 3-value output are all laid one uponanother. Even when the inclined print head causes a positionaldisplacement of the nozzles, some overlap is still expected andeffectively prevents quality reduction of the resulting image. Thismeans that accumulation of sheet feed errors does not cause much troublewhen an identical dot position can be scanned a plurality of times tooverlap two dots. This arrangement also ensures ‘solid’ filling.

As described previously, the arrangement of this embodiment enablesprinting by the fixed pitch sub-scans in the same manner as the priorart, thereby advantageously giving high-quality prints.

In this embodiment, dots may be superposed to effect the 3-value outputwith a time difference that is not shorter than the time period requiredfor one scan. This arrangement ensures sufficient drying of thepreviously formed dot and thereby prevents a blot of ink. Anotheradvantage is the improved dot density by superposing a new dot upon adried dot.

Although one embodiment of the present invention is described above, thepresent invention is not restricted to this embodiment in any sense. Forexample, in the above embodiment, the nozzle array arranged on the printhead includes an even nozzle array and an odd nozzle array that mutuallyinterpolate the nozzle interval, and the used nozzles are classified byselecting one every n nozzles in the main scanning direction.Alternatively, the print head may have an arrangement such that nozzlegroups each including n (=N) nozzles with a nozzle interval k in thesub-scanning direction are arranged at a fixed interval k in thesub-scanning direction. In the example of FIG. 2 where n is equal to 7,the seven nozzles may be aligned in the sub-scanning direction, like 7dots of #0-#6 and 7 dots of #7-#13. When the number of used nozzles n isselected among N nozzles in each nozzle group, selection of k and nwhich are prime to each other enables the superposition of dots of aspecific number equal to the number of nozzle groups through identicalcontrol.

C. Second Embodiment

A second embodiment of the present invention is described with thedrawings of FIGS. 14 through 18. In this embodiment, the like elementsof the first embodiment are assigned with the like numerals and notspecifically described here. This embodiment is characterized by thestructure with two nozzle arrays, that is, a nozzle array for spoutinghigher-density ink and a nozzle array for spouting lower-density ink, tosuperpose dots formed by ink droplets of different densities one uponthe other at an identical printing position, so as to ensure the richermulti-tone expression.

A print head 11 of this embodiment has a dark color nozzle array 12 forspouting higher-density ink (hereinafter referred to as ‘dark color’ andshown as ‘dark’ in the drawings) and a light color nozzle array 13 forspouting lower-density ink (hereinafter referred to as ‘light color’ andshown as ‘light’ in the drawings), which are arranged apart from eachother by a predetermined interval in the main scanning direction.

The dark color and light color here represent inks that have apractically identical color and different lightness (densities) and areselected for the multi-tone expression, for example, dark cyan and lightcyan or dark magenta and light magenta.

In this specification, the plural types of inks having a substantiallyidentical color and different densities are referred to as the‘different density inks’. The plural types of dots that are formed onthe printing paper (printing medium) and recognized by the observer tohave a substantially identical color but different print densities(reproduction densities) are referred to as the ‘different densitydots’. The observer generally recognizes the dots that are formed by thesame ink but have different diameters to have different print densities.It is accordingly possible to form the ‘different density dots’ by usingthe same ink of identical color and density while varying dot diameters.

Each of the nozzle arrays 12 and 13 has a first nozzle group thatincludes N nozzles arranged at a predetermined nozzle interval in thesub-scanning direction, and a second nozzle group that is arranged apartfrom the first nozzle group by a predetermined nozzle interval in thesub-scanning direction and includes N nozzles arranged at apredetermined nozzle interval in the sub-scanning direction.

The following describes the arrangement more in detail. As shown in FIG.15, the dark color nozzle array 12 has a first nozzle group 12A thatincludes five nozzles #5 to #9 shown by the symbol □ and arranged at apredetermined nozzle interval k in the sub-scanning direction, and asecond nozzle group 12B that is apart from the first nozzle group 12A bythe predetermined nozzle interval k and includes five nozzles #0 to #4shown by the symbol O and arranged at the predetermined nozzle intervalk in the sub-scanning direction. The dark color ink is spouted from therespective nozzles included in the nozzle groups 12A and 12B based onthe print image data.

In a similar manner, as shown in FIG. 16, the light color nozzle array13 has a first nozzle group 13A that includes five nozzles #5 to #9shown by the symbol ∇ and arranged at the predetermined nozzle intervalk in the sub-scanning direction, and a second nozzle group 13B that isapart from the first nozzle group 13A by the predetermined nozzleinterval k and includes five nozzles #0 to #4 shown by the symbol ⋄ andarranged at the predetermined nozzle interval k in the sub-scanningdirection. The light color ink is spouted from the respective nozzlesincluded in the nozzle groups 13A and 13B based on the print image data.In FIGS. 15 and 16, the hatched symbols of O, □, ∇, ⋄ represent thenozzles that can operate in printing.

In this embodiment, both the total number of nozzles N and the number ofused nozzles n are equal to ‘5’, and the values n and k are determinedto be prime to each other as described in the first embodiment. Forexample, k is set equal to ‘4’. These values N=n=5 and k=4 are onlyillustrative for the purpose of explanation, and the present inventionis not restricted to these values.

Like the data storage unit 6 in the first embodiment, the data storageunit 14 includes a memory, in which print image data includingmulti-value tone information is stored, and has a plurality of datablock areas suitable for the tone information. Since the print head 11used in this embodiment has the two nozzle arrays 12 and 13 for the darkcolor and the light color, the data storage unit 14 has four data blockareas, that is, raster blocks 0 to 3.

The two raster blocks 0, 1 are assigned to the dark color nozzle array12. The respective raster blocks 0, 1 represent four-value toneinformation by the 2 bits each assigned to one dot at an identicalposition. The 1-bit dot formation data to be output to the first nozzlegroup 12A is stored in the raster block 0, whereas the 1-bit dotformation data to be output to the second nozzle group 12B is stored inthe raster block 1.

When the dot formation data at a particular position in both the rasterblocks 0, 1 are equal to ‘0’, no dot is formed at the position. When thedot formation data in the raster block 0 is equal to ‘1’ and the dotformation data in the raster block 1 is equal to ‘0’, only one inkdroplet of the dark color hits the printing medium S, thereby forming adark color dot. When the dot formation data in both the raster blocks 0,1 are equal to ‘1’, two ink droplets of the dark color hit asubstantially identical position at an interval of a preset time period,thereby forming a darker color dot. This means that the 2-bitinformation at the corresponding positions in the raster blocks 0, 1enables the expression of the total 3 values: that is, no output ofdots, output of one dark color dot, and one overlapped dark color dot.

In a similar manner, the raster blocks 2 and 3, which representfour-value tone information by the 2 bits each assigned to one dot at anidentical position, are assigned to the light color nozzle array 13. The1-bit dot formation data to be output to the first nozzle group 13A isstored in the raster block 2, whereas the 1-bit dot formation data to beoutput to the second nozzle group 13B is stored in the raster block 3.The 2-bit information at the corresponding positions in the rasterblocks 2 and 3 enables the expression of the total 3 values: that is, nooutput of dots, output of one light color dot, and one overlapped lightcolor dot.

It is also possible to cause the light color nozzle array 13 to overlaya light color dot on a dark color dot which has already been formed bythe dark color nozzle array 12. The total of 8-value tones can thus beexpressed by the combinations of the superposable dark color dots withthe superposable light color dots. In this embodiment, however, 6-valuemulti-tone expression is applied as described later. The print headdriving unit 15 controls the dot outputs of the print head 11 based onthe dot formation data stored in these raster blocks 0 to 3.

An exemplified operation of the multi-value outputs by the respectivenozzle arrays 12 and 13 are described with FIGS. 15 and 16. FIG. 15shows the positions where the dark color nozzle array 12 forms dots by aplurality of main scan passes. The print head 11 is controlled by thedriving unit controller 14 so that the dot forming positions by thefirst nozzle group 12A overlap with those by the second nozzle group12B.

By way of example, the nozzle #8 in the first nozzle group 12A on thepass 1 and the nozzle #3 on the pass 5 are located at an identical dotforming position (raster line 1). As described in the first embodiment,dots are formed based on the 2-bit multi-value tone data stored in theraster blocks 0, 1. In the illustrated example, the overlapping of thedot forming positions (raster lines) occurs at a predetermined passinterval ΔP, that is, once every 4 passes.

As shown by the raster lines 1 to 23, dots can be initially formed onall the raster lines in a printing area by the nozzles in the precedingfirst nozzle group 12A. The nozzles in the following second nozzle group12B can subsequently superpose dots upon the initially formed dots. Whentwo nozzle groups, each having a plurality of nozzles arranged at apredetermined nozzle interval k in the sub-scanning direction, areadjoined to each other across the predetermined interval k in thesub-scanning direction, one nozzle group may be referred to as the‘preceding nozzle group’ and the other as the ‘following nozzle group’.

Referring to FIG. 16, like the dark color nozzle array 12, the lightcolor nozzle array 13 is controlled by the driving-unit controller 5 sothat the dot forming positions by the first nozzle group 13A overlapwith those by the second nozzle group 13B. In the light color nozzlearray 13, dots can be formed first by the first nozzle group 13A andthen by the second nozzle group 13B as shown in FIG. 16.

FIG. 17 shows the sequence of forming dots by the dark color nozzlearray 12 and the light color nozzle array 13.

As described above, the first nozzle group can form dots at specific dotforming positions, whereas the second nozzle group of the same nozzlearray can form dots at the same dot forming positions after thepredetermined pass interval ΔP (ΔP=4 in this embodiment). Referring toFIG. 17, the difference between the time point of dot forming by thepreceding first nozzle group and that by the following second nozzlegroup is equal to a time period TΔP, which depends upon the passinterval ΔP and the main scanning speed. The difference between the timepoint of dot formation by the corresponding nozzle groups of thedifferent nozzle arrays, on the other hand, is equal to a time periodTd, which depends upon a distance d between the nozzle arrays 12 and 13in the main scanning direction and the main scanning speed.

The sequence of possible dot formation at a specific dot formingposition is: preceding dark color dots (□) by the first nozzle group 12Ain the dark color nozzle array 12;→preceding light color dots (∇) by thefirst nozzle group 13A in the light color nozzle array 13;→followingdark color dots (O) by the second nozzle group 12B in the dark colornozzle array 12;→following light color dots (⋄) by the second nozzlegroup 13B in the light color nozzle array 13.

This sequence of forming the dark color dots and the light color dotsmay be utilized to effect, for example, 6-value multi-tone expression.FIG. 18 shows the relationship of: the 6-value tones in the range of 0to 5, the selected ink densities, the dot formation data stored in theraster blocks, and the conceptual plan view of the dots formed on theprinting medium S.

If the tone value is zero, which represents no output of dots at aspecific position, the dot formation data ‘0’ is given to thecorresponding nozzles in the respective nozzle arrays 12 and 13. No inkdroplets are accordingly spouted from either nozzles to form pixels.

If the tone value is 1, only one light color dot (∇) is formed. Eitherthe first nozzle group 13A or the second nozzle group 13B is actuated tospout one ink droplet of the light color, in order to form only onelight color dot. It is accordingly sufficient to give the dot formationdata ‘1’ to either one of the corresponding nozzles in the respectivenozzle groups. By taking into account the case in which a light colordot is superposed as discussed later, however, it is advantageous togive the data ‘1’ to the nozzle in the preceding first nozzle group 13Awhile giving the data ‘0’ to the corresponding nozzle in the followingsecond nozzle group 13B. Namely the preceding first nozzle group 13Aform a light color dot to effect the tone value 1.

If the tone value is 2, another light color dot (⋄) is superposed uponthe light color dot (∇) formed by the preceding first nozzle group 13Aafter the predetermined pass interval ΔP. The light color dot formed bythe preceding nozzle is sufficiently dried before the elapse of the passinterval ΔP, so that superposing another ink droplet by the followingnozzle does not cause a significant blot of the resulting dot. Since anew light color dot is superposed upon the light color dot previouslyformed after it dried, the density of the resulting dot is increasedcompared with a single light color dot.

The tone value of 3 is attained by a single dark color dot (□). In thesame manner as the case of the tone value of 1, the dot formation data‘1’ is given to only the nozzle in the preceding first nozzle group 12A.This causes only one ink droplet of the dark color to hit a specifiedposition, so as to effect the tone value 3, which represents the higherdensity than that of the tone value 2.

The tone value of 4 is attained by superposing a light color dot upon adark color dot. As discussed with FIG. 17, there are three availablemethods applicable to superpose a light color dot upon a dark color dot.

The first method first forms a preceding dark color dot (□) by the firstnozzle group 12A of the dark color nozzle array 12 and then forms apreceding light color dot (∇) by the first nozzle group 13A of the lightcolor nozzle array 13 (□+∇). The second method first forms a followingdark color dot (O) by the second nozzle group 12B of the dark colornozzle array 12 and then forms a following light color dot (⋄) by thesecond nozzle group 13B of the light color nozzle array 13 (O+⋄). Thethird method first forms a preceding dark color dot (□) by the firstnozzle group 12A of the dark color nozzle array 12 and then forms afollowing light color dot (⋄) by the second nozzle group 13B of thelight color nozzle array 13 (□+⋄). In both the first and second methods,the jetting interval between ink droplets is the extremely short timeperiod Td, which depends upon the nozzle array interval d. There isaccordingly a possibility that a following dot is formed before thepreceding dot is sufficiently dried.

In this embodiment, the third method is applied so that a following dotis superposed upon the preceding dot that has been dried sufficiently.The third method applied to this embodiment effectively prevents an inkblot and increases the density of the resulting dot. Both the first andsecond methods are, however, included in the technical scope of thepresent invention.

The tone value of 5 is attained by overlapping two dark color dots eachother. In the same manner as the case of the tone value of 2, afollowing dark color dot is formed after the time period TΔP, whichdepends upon the pass interval Δ, has elapsed since formation of apreceding dark color dot. This increases the density (tone) of theresulting dot, compared with a single dark color dot.

As described above, the second embodiment enables inks of differentdensities to be spouted at an identical position so that dots ofdifferent densities overlap each other. Compared with the firstembodiment, the second embodiment ensures richer tone expression andcarries out high-quality printing like a photographic image.

Since the second embodiment can overlap dots at an identical position,like the first embodiment, it ensures formation of a dot having a nearlycomplete round shape if the accuracy of the main scans and the sub-scansis within a predetermined range. This improves the deterioration of thegranularity in the low-density area due to the uneven dot shape. Evenwhen the accuracy of the sub-scans is lowered by the effects of thepaper quality or the humidity, the superposed dots tend to grow in thesub-scanning direction, and a white streak (white banding phenomenon) isprevented accordingly. If dots grow in the sub-scanning direction, adecrease in overlapping area of the dots reduces the density at theprinted position from the expected density. The growth of the dots inthe sub-scanning direction, however, increases the dot formation area.This increase in dot formation area compensates for the reduced densityand thereby prevents the printing quality from being lowered.

The structure of making a preceding dot and a following dot overlap eachother after the predetermined pass interval ΔP enables a new dot to besuperposed upon the dot previously formed and sufficiently dried. Thiseffectively prevents a blot of the resulting dot on the sheet surfacewhile increasing the density of the resulting dot, thereby increasingthe amount of ink hit per unit area. This extends the range of toneexpression per unit area and improves the degree of freedom of dots inmiddle tone.

Although the second embodiment is directed to the case where the inkdensity is classified into two levels, that is, dark and light, thepresent invention is not restricted to this structure but is applicableto any other structures, for example, one classifying the ink densityinto three levels, that is, high density, intermediate density, and lowdensity.

In the ink jet printer for color printing, the different density inksmay be provided for the four colors, black, cyan, magenta, and yellow,or for the three colors, cyan, magenta, and yellow. Alternatively thedifferent density inks may be provided only for one or a plurality ofspecific colors. For example, the different density inks may be providedfor only cyan and magenta, while the ink of a single density is used forblack and yellow.

although the above embodiment uses two nozzle groups for the dark inkand the light ink, the present invention is also applicable to the casein which only one nozzle group is used for the dark ink and the lightink, respectively. This configuration is attained by specifying one ofthe two nozzle arrays 2 a and 2 b for the dark ink and the other for thelight ink in the structure of the first embodiment shown in FIG. 9. Inthis case, the multi-levels expressible by one pixel include a firsttone level obtained by one dot of the light ink, a second tone levelobtained by one dot of the dark ink, and a third tone level obtained byoverlapping dots of the dark ink and the light ink.

The present invention is also applicable to the case in which anidentical ink is used to form plural types of different density dotshaving different sizes, so as to form multi-level dots. In this case, atleast one nozzle group is used for each of the plural types of differentdensity dots having different sizes. The different density dots havingdifferent sizes can be formed by, for example, a nozzle group of arelatively large diameter and a nozzle group of a relatively smalldiameter. These dots of different sizes can alternatively be formed bythe technique of dot diameter modulation where the dot diameter (thatis, the spouted ink droplet) is varied by changing the ink spoutingenergy to at least one of a plurality of nozzle groups.

D. Method of Sub-Scan Feed

A variety of scanning schemes with a plurality of different sub-scanfeed amounts may be applicable to the respective nozzle groups in thefirst and the second embodiments discussed above. The followingdescribes the fundamental conditions required for the general scanningscheme, prior to the explanation for the variety of scanning schemesapplied to the embodiments of the present invention.

FIG. 19 shows the fundamental conditions of the general scanning scheme.FIG. 19(A) shows a sub-scan feed with one nozzle group including fournozzles, and FIG. 19(B) shows the parameters of this scanning scheme.The details of the parameters will be described later. The followingdescription is made for the case in which one nozzle group is used forspouting identical ink. For example, the nozzle group including fournozzles shown in FIG. 19(A) corresponds to either the even nozzle array2 a or the odd nozzle array 2 b of FIG. 9.

FIGS. 19(A) and 19(B) show basic conditions of a general scanning schemewhen the number of scan repeats s is equal to one. FIG. 19(A)illustrates an example of sub-scan feeds with five nozzles, and FIG.19(B) shows parameters of the scanning scheme. In the drawing of FIG.19(A), solid circles including numerals indicate the positions of thefive nozzles in the sub-scanning direction after each sub-scan feed. Theencircled numerals 0 through 3 denote the nozzle numbers. The fivenozzles are shifted in the sub-scanning direction every time when onemain scan is concluded. Actually, however, the sub-scan feed is executedby feeding a printing paper with the sheet feed motor 23 (FIG. 5).

As shown on the left-hand side of FIG. 19(A), the sub-scan feed amount Lis fixed to four dots. On every sub-scan feed, the four nozzles areshifted by four dots in the sub-scanning direction. When the number ofscan repeats s is equal to one, each nozzle can record all dots (pixels)on the raster line. The right-hand side of FIG. 19(A) shows the nozzlenumbers of the nozzles which record dots on the respective raster lines.There are non-serviceable raster lines above or below those raster linesthat are drawn by the broken lines, which extend rightward (in the mainscanning direction) from a circle representing the position of thenozzle in the sub-scanning direction. Recording of dots is thusprohibited on these raster lines drawn by the broken lines. On thecontrary, both the raster lines above and below a raster line that isdrawn by the solid line extending in the main scanning direction arerecordable with dots. The range in which all dots can be recorded ishereinafter referred to as the “effective record area” (or the“effective print area”). The range in which the nozzles scan but all thedots cannot be recorded are referred to as the “non-effective recordarea (or the “non-effective print area)”. All the area which is scannedwith the nozzles (including both the effective record area and thenon-effective record area) is referred to as the nozzle scan area.

Various parameters related to the scanning scheme are shown in FIG.19(B). The parameters of the scanning scheme include the nozzle pitch k[dots], the number of used nozzles n, the number of scan repeats s,number of effective nozzles Neff, and the sub-scan feed amount L [dots].The nozzle pitch k [dots] indicates how many pitches (dot pitches) inthe resulting recorded image correspond to the interval between thecenter points of the nozzles on the print head. In the example of FIG.19, k is equal to 3. The number of used nozzles n denotes the number ofnozzles actually used for dot formation among all the nozzles mounted onthe print head. In the example of FIG. 19, n is equal to 4.

When the nozzles arranged in zigzag (FIG. 2) are divided into the twonozzle groups, that is, the even nozzle group #0, #2, . . . , #14 andthe odd nozzle group #1, #3, . . . , #15, as described in the firstembodiment, the nozzle pitch 2 k in each nozzle group shown in FIG. 2corresponds to the nozzle pitch k in FIG. 19.

The number of scan repeats s indicates how many passes (main scans) arerequired to fill each main scanning line with dots. The number of scanrepeats s also means that dots are formed intermittently once every sdots during one main scan. The number of scan repeats s is accordinglyequal to the number of nozzles used for recording all the dots on therespective main scanning lines. In the description hereinafter, the mainscanning line is referred to as the ‘raster line’. In the example ofFIG. 19, s is equal to 1 because each raster line is filled by one pass.As will be described later, when s is equal to or greater than 2, dotsare formed intermittently in the main scanning direction. The number ofeffective nozzles neff is obtained by dividing the number of usednozzles n by the number of scan repeats s. The number of effectivenozzles neff may be regarded as the net number of raster lines that canbe fully recorded during a single main scan. The meaning of the numberof effective nozzles neff will be further discussed later.

The table of FIG. 19(B) shows the sub-scan feed amount L, itsaccumulated value ΣL, and a nozzle offset F after each sub-scan feed.The offset F is a value indicating the distance in number of dotsbetween the nozzle positions and reference positions of offset 0. Thereference positions are presumed to be those periodic positions whichinclude the initial positions of the nozzles where no sub-scan feed hasbeen conducted (every fourth dot in FIG. 19(A)). For example, as shownin FIG. 19(A), a first sub-scan feed moves the nozzles in thesub-scanning direction by the sub-scan feed amount L (4 dots). Thenozzle pitch k is 3 dots as mentioned above. The offset F of the nozzlesafter the first sub-scan feed is accordingly 1 (see FIG. 19(A)).Similarly, the position of the nozzles after the second sub-scan feed isΣL(=8) dots away from the initial position so that the offset F is 2.The position of the nozzles after the third sub-scan feed is ΣL(=12)dots away from the initial position so that the offset F is 0. Since thethird sub-scan feed brings the nozzle offset F back to zero, all dots ofthe raster lines within the effective record area can be serviced byrepeating the cycle of 3 sub-scans.

As will be understood from the above example, when the nozzle positionis apart from the initial position by an integral multiple of the nozzlepitch k, the offset F is zero. The offset F is given by (ΣL)%k, where ΣLis the accumulated value of the sub-scan feed amount L, k is the nozzlepitch, and “%” is an operator indicating that the remainder of thedivision is taken. Viewing the initial position of the nozzles as beingperiodic, the offset F can be viewed as an amount of phase shift fromthe initial position.

When the number of scan repeats s is one, the following conditions arerequired to avoid skipping or overwriting of raster lines in theeffective record area:

Condition c1: The number of sub-scan feeds in one feed cycle is equal tothe nozzle pitch k.

Condition c2: The nozzle offsets F after the respective sub-scan feedsin one feed cycle assume different values in the range of 0 to (k−1).

Condition c3: Average sub-scan feed amount (ΣL/k) is equal to the numberof used nozzles n. In other words, the accumulated value ΣL of thesub-scan feed amount L for the whole feed cycle is equal to a product(n×k) of the number of used nozzles n and the nozzle pitch k.

The above conditions can be understood as follows. Since (k−1) rasterlines are present between adjoining nozzles, the number of sub-scanfeeds required in one feed cycle is equal to k so that the (k-1) rasterlines are serviced during one feed cycle and that the nozzle positionreturns to the reference position (the position of the offset F equal tozero) after one feed cycle. If the number of sub-scan feeds in one feedcycle is less than k, some raster lines will be skipped. If the numberof sub-scan feeds in one feed cycle is greater than k, on the otherhand, some raster lines will be overwritten. The first condition c1 isaccordingly required.

If the number of sub-scan feeds in one feed cycle is equal to k, therewill be no skipping or overwriting of raster lines to be recorded onlywhen the nozzle offsets F after the respective sub-scan feeds in onefeed cycle take different values in the range of 0 to (k−1). The secondcondition c2 is accordingly required.

When the first and the second conditions c1 and c2 are satisfied, eachof the n nozzles records k raster lines in one feed cycle. Namely n×kraster lines can be recorded in one feed cycle. When the third conditionc3 is satisfied, the nozzle position after one feed cycle (that is,after the k sub-scan feeds) is away from the initial position by the n×kraster lines as shown in FIG. 19(A). Satisfying the above first throughthe third conditions c1 to c3 thus prevents skipping or overwriting ofraster lines to be recorded in the range of n×k raster lines.

FIGS. 20(A) and 20(B) show the basic conditions of a general scanningscheme when the number of scan repeats s is no less than 2. When thenumber of scan repeats s is 2 or greater, each raster line is recordedwith s different nozzles. In the description hereinafter, the scanningscheme adopted when the number of scan repeats s is not less than 2 isreferred to as the “overlap scheme”.

The scanning scheme shown in FIGS. 20(A) and 20(B) amounts to thatobtained by changing the number of scan repeats s and the sub-scan feedamount L among the scanning scheme parameters shown in FIG. 19(B). Aswill be understood from FIG. 20(A), the sub-scan feed amount L in thescanning scheme of FIGS. 20(A) and 20(B) is a constant value of twodots. In FIG. 20(A), the nozzle positions after the odd-numberedsub-scan feeds are indicated by the diamonds. As shown on the right-handside of FIG. 20(A), the dot positions recorded after the odd-numberedsub-scan feed are shifted by one dot in the main scanning direction fromthe dot positions recorded after the even-numbered sub-scan feed. Thismeans that the plurality of dots on each raster line are recordedintermittently by each of two different nozzles. For example, theupper-most raster in the effective record area is intermittentlyrecorded on every other dot by the No. 2 nozzle after the first sub-scanfeed and then intermittently recorded on every other dot by the No. 0nozzle after the fourth sub-scan feed. In the overlap scheme, eachnozzle is generally driven at an intermittent timing so that recordingis prohibited for (s−1) dots after recording of one dot during a singlemain scan.

In the overlap scheme, the multiple nozzles used for recording the sameraster line are required to record different positions shifted from oneanother in the main scanning direction. The actual shift of recordingpositions in the main scanning direction is thus not restricted to theexample shown in FIG. 20(A). In one possible scheme, dot recording isexecuted at the positions indicated by the circles shown in theright-hand side of FIG. 20(A) after the first sub-scan feed, and isexecuted at the shifted positions indicated by the diamonds after thefourth sub-scan feed.

The lower-most row of the table of FIG. 20(B) shows the values of theoffset F after each sub-scan feed in one feed cycle. One feed cycleincludes six sub-scan feeds. The offsets F after each of the sixsub-scan feeds assume every value between 0 and 2, twice. The variationin the offset F after the first through the third sub-scan feeds isidentical with that after the fourth through the sixth sub-scan feeds.As shown on the left-hand side of FIG. 20(A), the six sub-scan feedsincluded in one feed cycle can be divided into two sets of sub-cycles,each including three sub-scan feeds. One feed cycle of the sub-scanfeeds is completed by repeating the sub-cycles s times.

When the number of scan repeats s is an integer of not less than 2, thefirst through the third conditions c1 to c3 discussed above arerewritten into the following conditions c1′ through c3′:

Condition c1′: The number of sub-scan feeds in one feed cycle is equalto a product (k×s) of the nozzle pitch k and the number of scan repeatss.

Condition c2′: The nozzle offsets F after the respective sub-scan feedsin one feed cycle assume every value between 0 to (k−1), s times.

Condition c3′: Average sub-scan feed amount {ΣL/(k×s)} is equal to thenumber of effective nozzles neff (=n/s). In other words, the accumulatedvalue ΣL of the sub-scan feed amount L for the whole feed cycle is equalto a product {neff×(k×s)} of the number of effective nozzles neff andthe number of sub-scan feeds (k×s).

The above conditions c1′ through c3′ hold even when the number of scanrepeats s is one. This means that the conditions c1′ through c3′generally hold for the scanning scheme irrespective of the number ofscan repeats s. When these three conditions c1′ through c3′ aresatisfied, there is no skipping or overwriting of dots recorded in theeffective record area. If the overlap scheme is applied (if the numberof scan repeats s is not less than 2), the recording positions on thesame raster should be shifted from each other in the main scanningdirection.

Partial overlapping may be applied for some scanning schemes. In the“partial overlap” scheme, some raster lines are recorded by one nozzleand other raster lines are recorded by multiple nozzles. The number ofeffective nozzles neff can be also defined in the partial overlapscheme. By way of example, if two nozzles among four used nozzlescooperatively record one identical raster line and each of the other twonozzles records one raster line, the number of effective nozzles neff is3. The three conditions c1′ through c3′ discussed above also hold forthe partial overlap scheme.

It may be considered that the number of effective nozzles neff indicatesthe net number of raster lines recordable in a single main scan. Forexample, when the number of scan repeats s is 2, n raster lines can berecorded by two main scans where n is the number of actually-usednozzles. The net number of raster lines recordable in a single main scanis accordingly equal to n/S (that is, neff). The number of effectivenozzles neff in this embodiment corresponds to the number of effectivedot forming elements in the present invention.

FIG. 21 shows a first scanning scheme in the embodiment of the presentinvention. The scan parameters of this scanning scheme are shown in thebottom of FIG. 21, where the nozzle pitch k is equal to 4 dots, thenumber of used nozzles n is equal to 8, the number of scan repeats s isequal to 1, and the number of effective nozzles neff is equal to 8.

In the example of FIG. 21, nozzle numbers #0 through #7 are allocated tothe eight used nozzles from the top. In the first scanning scheme, foursub-scan feeds constitute one cycle, and the amount of the sub-scan feedL is varied in the sequence of 10, 7, 6, and 9 dots. This means that aplurality of different values are used for the sub-scan feed amount L.The positions of the eight nozzles in the respective sub-scan feeds areshown by four different figures. The right end of FIG. 21 shows by whichnozzle and after which sub-scan feed the dots on the raster lines in theeffective record area are to be recorded. In the first scanning scheme,a non-effective record area of 20 raster lines is present before theeffective record area. Namely the effective record area starts at the21st raster line from the upper end of the nozzle scan area (the rangeincluding the effective record area and the non-effective record area).The nozzle position in the first main scan is set to be apart from theupper end of the printing paper by a predetermined distance. The earlierstarting position of the effective record area enables the dots to berecorded from the position closer to the upper end of the printingpaper.

FIGS. 22(A) and 22(B) show the scan parameters and the raster numbers ofthe effective raster lines recorded by the respective nozzles in thefirst scanning scheme. The table of FIG. 22(A) shows the sub-scan feedamount L and its summation ΣL for each sub-scan feed, the offset F ofthe nozzle after each sub-scan feed, and the offset G of the sub-scanfeed amount L. The offset G of the sub-scan feed amount L is theremainder obtained by dividing the sub-scan feed amount L by the nozzlepitch k. The meaning of the offset G of the sub-scan feed amount L willbe described later in detail.

The parameters shown in FIG. 22(A) satisfy the three conditions c1′through c3′ discussed above. The number of sub-scan feeds in one cycleis equal to the product (k×s=4) of the nozzle pitch k(=4) and the numberof scan repeats s(=1) (first condition c1′). The offset F of the nozzleafter each sub-scan feed in one cycle assumes the values in the range of0 to (k−1) (i.e., in the range of 0 to 3) (second condition c2′). Theaverage sub-scan feed amount (ΣL/k) is equal to the number of effectivenozzles neff(=8) (third condition c3′). The first scanning schemeaccordingly satisfies the fundamental requirement that there is nodropout or overlap of recorded raster lines in the effective recordingarea.

The first scanning scheme also has the following two features. The firstfeature is that the nozzle pitch k and the number of used nozzles n areintegers which are no less than 2 and which are not relatively prime.The second feature is that a plurality of different values are used forthe sub-scan feed amount L. As discussed previously in the prior art,the conventional scanning scheme sets the number of nozzles n and thenozzle pitch k at the integers that are relatively prime. The number ofnozzles n actually used among a large number of nozzles provided is thusrestricted to the value that is prime to the nozzle pitch k. In otherwords, the problem of the conventional process is that the nozzlesprovided are not sufficiently used in many cases. Application of thescanning scheme having the first feature that the nozzle pitch k and thenumber of used nozzles n are integers which are no less than 2 and whichare not relatively prime, on the other hand, advantageously increasesthe number of used nozzles as many as possible. The second featureallows the fundamental requirement that there is no dropout or overlapof recorded raster lines in the effective record area to be satisfiedwhen the scanning scheme has the first feature. There will be dropout oroverlap of raster lines if the scanning scheme that has the firstfeature and a fixed sub-scan feed amount L is applied.

The scanning scheme using a plurality of different sub-scan feed amountsis applicable not only to the case in which the nozzle pitch k and thenumber of used nozzles n are integers of not less than 2 that are notrelatively prime, but to the case in which the nozzle pitch k and thenumber of used nozzles n are prime to each other.

FIG. 22(B) shows the raster numbers of the effective raster linesrecorded by the respective nozzles in the main scan after each sub-scanfeed in the first scanning scheme. The left-hand side of FIG. 22(B)shows the nozzle numbers #0 through #7. The values on the right-handside of the nozzle numbers represent which raster lines in the effectiverecord area are recorded by the respective nozzles after the 0th to 7thsub-scan feeds. By way of example, in the main scan after the 0thsub-scan feed (that is, in the first main scan for recording theeffective record area), the nozzles #5 through #7 record the 1st, 5th,and 9th effective raster lines. In the main scan after the 1st sub-scanfeed, the nozzles #3 through #7 record the 3rd, 7th, 11th, 15th, and19th effective raster lines. The term “effective raster lines” heredenotes the raster lines in the effective record area.

It can be understood that, in FIG. 22(B), a difference between rasternumbers of the effective raster lines recorded during one main scan isequal to the nozzle pitch k(=4). One scan cycle accordingly records n×k(that is, 32) raster lines. Since any successive nozzles are apart fromeach other by the nozzle pitch k, one cycle does not record 32sequential raster lines as clearly understood from FIG. 21. FIG. 22(B)shows which nozzles are used to record the first 32 raster lines in theeffective record area.

In FIG. 22(B), the effective raster numbers written in the brackets showthat the raster lines at the positions having the equivalent scanningconditions have been recorded in the previous cycle. Namely thedifference obtained by subtracting 32 from the numeral in the bracketsindicates the equivalent raster line number. For example, the rasterline of the effective raster number 36 recorded by the nozzle #0 ispresent at the position having the equivalent scanning conditions tothose of the raster line of the effective raster number 4.

FIG. 23 shows the nozzle numbers for recording the effective rasterlines in the first scanning scheme. The numerals 1 through 31 on theleft-end column of FIG. 23 show the effective raster numbers. Theright-hand side of FIG. 23 shows the positions of the effective rasterlines recorded by the eight nozzles #0 through #7 in the main scansafter the respective sub-scan feeds. For example, in the main scan afterthe 0th sub-scan feed, the nozzles #5 through #7 record the 1st, 5th,and 9th effective raster lines, respectively. Comparison between FIG. 23and FIG. 22(B) clearly shows the relationship between the effectiveraster lines and the nozzle numbers.

Four different symbols “·”, “x”, “↑”, and “↓” in the second-left columnof FIG. 23 show whether or not the adjoining raster lines have alreadybeen recorded before the recording of each raster line. The respectivesymbols have the following meaning:

↓: Only one raster line immediately below itself has already beenrecorded.

↑: Only one raster line immediately above itself has already beenrecorded.

x: Both raster lines above and below itself have already been recorded.

·: Neither of the raster lines above and below itself have beenrecorded.

The recording state of the adjoining raster lines above and below eachraster line affects the image quality of the raster line being recorded.The effects on the image quality are ascribed to the dryness of ink onthe adjoining raster lines that have already been recorded and tosub-scan feed errors. If the pattern by the four different symbolsappears at a relatively large interval, it may deteriorate the imagequality of the whole image. In the first scanning scheme shown in FIG.23, however, the pattern by the four different symbols does not show anyclear periodicity. It is accordingly expected that the first recordingscheme causes less deterioration of the image quality due to this reasonbut enables an image of relatively high image quality to be recorded.

The third-left column of FIG. 23 shows the value Δ representing how manysub-scan feeds have been executed at the maximum between recording ofeach raster line and recording of the adjoining raster line. The value Δis hereinafter referred to as the “sub-scan feed number difference”. Byway of example, the second effective raster line is recorded by thenozzle #1 after the 2nd sub-scan feed, whereas the first raster line isrecorded by the nozzle #5 after the 0th sub-scan feed and the thirdraster line is recorded by the nozzle #3 after the 1st sub-scan feed.The sub-scan feed number difference Δ is accordingly equal to 2 withrespect to the second raster line. In a similar manner, the fourthraster line is recorded after three sub-scan feeds have been executedsince recording of the fifth raster line. The sub-scan feed numberdifference Δ is thus equal to 3 with respect to the fourth raster line.

Since one cycle consists of k(=4) sub-scan feeds, the sub-scan feednumber difference Δ may be the value in the range of 0 to k. In thefirst scanning scheme for k=4, it is understood that the maximumsub-scan feed number difference Δ is equal to 3, which is smaller thanthe possible upper limit value k(=4).

It is ideal that the sub-scan feed is carried out strictly by the amountequal to an integral multiple of the dot pitch. In the actual state,however, the sub-scan feed has some feeding error. The sub-scan feederror is accumulated at every time of sub-scan feed. When a large numberof sub-scan feeds are interposed between recording of adjoining tworaster lines, the accumulated sub-scan feed error may cause a positionalmisalignment of the adjoining two raster lines. As mentioned above, thesub-scan feed number difference Δ shown in FIG. 23 denotes the number ofsub-scan feeds carried out between recording of the adjoining rasterlines. The smaller sub-scan feed number difference Δ is preferable, inorder to minimize the positional misalignment of the adjoining rasterlines due to the accumulated sub-scan feed error. In the first scanningscheme for k=4 shown in FIG. 23, the sub-scan feed number difference Δis not greater than 3 and is smaller than the upper limit value 4. Thisallows a favorable, image to be recorded from this viewpoint.

The first scanning scheme described above may be applied to drive theprint head 2 (see FIG. 9) in the first embodiment as well as to drivethe print head 11 (see FIG. 14) in the second embodiment. It should,however, be noted that the scanning parameters in the first scanningscheme relate to one nozzle group (either the even nozzle array or theodd nozzle array in the first embodiment). The dot recording processesof the first and second embodiments described above are characterized bythe procedure of forming each pixel, and the first and the secondembodiments are thus arbitrarily applicable to the cases of differentsettings for the sub-scan feed amounts L in the scanning scheme and thedifferent recording sequence of the respective pixels on an identicalraster line. The first and second embodiments are also applicable to avariety of other scanning schemes described below.

FIG. 24 shows the scan parameters and the raster numbers of theeffective raster lines recorded by the respective nozzles in a thirdscanning scheme using plural values of sub-scan feed amounts. In thesecond scanning scheme, the nozzle pitch k is equal to 8 dots and thenumber of used nozzles n is equal to 16. The number of scan repeats isequal to 1. Like the first scanning scheme, the second scanning schemehas the first feature that the nozzle pitch k and the number of usednozzles n are integers which are no less than 2 and which are notrelatively prime, and the second feature that a plurality of differentvalues are used for the sub-scan feed amount L.

FIG. 25 shows the nozzle numbers for recording the effective rasterlines in the second scanning scheme. In the second scanning scheme, thepattern of the symbols @ representing the recording state of theadjoining raster lines above and below each raster line does not have asignificantly large period. It is accordingly expected to attain therelatively high image quality. The difference in number of sub-scanfeeds Δ is equal to either 3 or 5, which is significantly smaller thanthe possible upper limit 8. This arrangement reduces the accumulatederror of sub-scan feed and thereby enables a favorable image to berecorded.

In addition to the two features discussed above, the second scanningscheme has another feature with respect to the sub-scan feed amount L.In the second scanning scheme, the sub-scan feed amount L assumes valuesof 13 and 21 and the offset G (=L%k) of the sub-scan feed amount L is aconstant value as shown in the table of FIG. 24(A). The offset G denotesa deviation of the periodical positions (that is, the phase deviation)of the plurality of nozzles after a sub-scan feed from the periodicalpositions of these nozzles before the sub-scan feed. For example, whenthe offset G is equal to zero (that is, when the sub-scan feed amount Lis an integral multiple of the nozzle pitch k), the periodical positionsof the nozzles after the sub-scan feed overlaps the periodical positionof the nozzles before the sub-scan feed. In order to avoid such anoverlap, the offset G is generally not equal to zero. According to theperiodicity of the arrangement of the nozzles, the fixed offset G withrespect to the sub-scan feed amount L causes the nozzles to be fed by afixed amount of shift in the sub-scanning direction. By way of example,when the offset G is equal to 1, the nozzles will be arranged at thepositions whose phase is shifted downward by one raster line from thenozzle positions before the sub-scan feed.

The offset G of the sub-scan feed amount L will not be equal to zero inany case. As clearly understood from the definition of the offset G, thevalue of the offset G is smaller than the nozzle pitch k. Especiallywhen the offset G is constant, the offset G is set at an integer that isrelatively prime to the nozzle pitch k. Such setting enables thecondition c2′ discussed above, that is, ‘The offset F of the nozzlesafter each sub-scan feed included in one cycle takes a value in therange of 0 to k−1) and the value is repeated s times.’, to be satisfied.A desirable value for the constant offset G of the sub-scan feed amountL is determined by considering the following factors.

FIG. 26 shows an example of the scanning scheme when the offset G isfixed to one. In this example, the raster line 9 is recorded after afirst sub-scan feed in the effective recording area. The raster line 8is recorded after seven sub-scan feeds since then. The errors of k timesof sub-scan feeds re accordingly accumulated between these two rasterlines. The raster lines 18 and 17 hold a similar relation. With a viewto preventing the error of sub-scan feed from being accumulated, it isdesirable to set the sub-scan feed amount L in such a manner that theoffset G of the sub-scan feed amount L has a value other than 1. Likethe case of G=1, in the case of the offset G equal to (k−1), the errorof k sub-scan feeds is accumulated. It is accordingly desirable to setthe offset G equal to a value other than (k−1).

In the example of FIG. 26, the pattern of the symbols @ representing therecording state of the adjoining raster lines above and below eachraster line shows a significantly large cycle. It is accordinglypossible that a pattern of the large cycle is observed in a recordedimage. In order to prevent the periodic pattern from appearing, it ispreferable that the constant offset G is set at a value other than 1 and(k−1) to.

When taking into account of the above factors, the constant offset G ispreferably set at a value which is prime to the nozzle pitch k and inthe range of 2 to (k−2) when the offset G of the sub-scan feed amount Lis fixed to a constant value. FIG. 27 shows preferable combinations ofthe nozzle pitch k and the offset G of the sub-scan feed amount. Thevalues shown in FIG. 27 all satisfy the conditions of the desirableoffset G.

When the offset G is equal to either 1 or (k−1), adjoining raster linesare recorded in a successive manner. In this case, before the ink isdried on a raster line just recorded, the recording on an adjoiningraster line starts, thereby causing a blur of ink. The similarphenomenon occurs not only when the offset G has a constant value butthe offset G is varied for each sub-scan feed amount L. In order toprevent the blur of ink, whether or not the offset G of the sub-scanfeed amount L is constant, it is preferable to set the sub-scan feedamount L so that the offset G takes a value other than 1 and (k−1).

In the second scanning scheme, the plurality of values (13 and 21) areused for the sub-scan feed amount L, and the offset G of the sub-scanfeed amount L is a preferable constant value. This arrangementeffectively prevents the accumulation of the sub-scan feed errors,thereby enabling an image of high image quality to be recorded.

FIGS. 28(A) and 28(B) show the scan parameters and the raster numbers ofthe effective raster lines recorded by the respective nozzles in a thirdscanning scheme using plural values of sub-scan feed amounts. Thedifference between the third scanning scheme and the second scanningscheme shown in FIGS. 24(A) and 24(1) is only the sub-scan feed amountL. Like the second scanning scheme, the third scanning scheme has thefirst feature that the nozzle pitch k and the number of used nozzles nare integers which are no less than 2 and which are not relativelyprime, and the second feature that a plurality of different values areused for the sub-scan feed amount L. The third scanning scheme also hasthe third feature that the offset G (=L%k) of the sub-scan feed amount Lis a constant value. As shown in FIG. 27 discussed above, the value (=5)of the offset G of the sub-scan feed amount L in the third scanningscheme is an especially preferable one.

FIG. 29 shows the nozzle numbers for recording the effective rasterlines in the third scanning scheme. Like the second scanning schemeshown in FIG. 25, in the third scanning scheme, the pattern of thesymbols @ representing the recording state of the adjoining raster linesabove and below each raster line does not have a significantly largecycle. It is accordingly expected to attain a relatively favorable imagequality. Since the difference in number of sub-scan feeds Δ is equal toeither 3 or 5, which is significantly smaller than the possible upperlimit 8, a favorable image can be recorded from the viewpoint of smalleraccumulated error of the sub-scan feed.

Having the variety of features that are substantially similar to thoseof the second scanning scheme, the third scanning scheme can record ahigh quality image in the same manner as the second scanning scheme.

FIG. 30 shows the scan parameters in a fourth scanning scheme usingplural values of sub-scan feed amounts. In the fourth scanning scheme,the nozzle pitch k is equal to 8 dots and the number of used nozzles nis equal to 32. The number of scan repeats s is equal to 2 and thenumber of effective nozzles neff is equal to 16. As clearly understoodfrom the comparison with the parameters in the third scanning schemeshown in FIG. 28, the number of effective nozzles neff in the fourthscanning scheme is kept equal to that in the third scanning scheme,whereas the number of scan repeats s is set equal to 2 and the number ofused nozzles n is doubled in the fourth scanning scheme. Since thenozzle pitch k and the number of effective nozzles neff in the fourthscanning scheme are equal to those in the third scanning scheme, thesame values as those of the third scanning scheme are used for thesub-scan feed amount L. However, since the eight sub-scan feeds shown inthe table of FIG. 30 records the raster lines only once, another eightsub-scan feeds are executed to record dots without any space. The eightsub-scan feeds shown in the table of FIG. 30 accordingly correspond tothe sub-cycle in FIG. 20(A) discussed previously.

FIG. 31 shows the raster numbers of the effective raster lines recordedby the respective nozzles in the fourth scanning scheme. The rasternumbers of FIG. 31 are similar to those of the third scanning schemeshown in FIGS. 28(A) and 28(B). The raster line with a negative numberrepresents that dots are recorded at the positions which are shifted byone dot in the main scanning direction on the raster line. FIG. 32 showsthe nozzle numbers for recording the effective raster lines in thefourth scanning scheme. In FIG. 32, the nozzle with a negative numberrepresents that the nozzle records dots at the positions which areshifted by one dot in the main scanning direction. As clearly understoodfrom the drawing, two nozzles of different numbers are positioned on thesame raster line, and the respective nozzles record dots at thepositions which are shifted by one dot in the main scanning direction onthe raster line. This enables all the dots in the effective recordingarea to be recorded. In general, s pieces of different nozzles (sdenotes the number of scan repeats) are positioned on the same rasterline, and the s pieces of nozzles respectively record dots at thepositions which are shifted from one another in the main scanningdirection on the raster line.

Having similar features to those of the third scanning scheme except forthe number of scan repeats s, the fourth scanning scheme can record ahigh quality image in the same manner as the third scanning scheme.

Although the above embodiments are concerned with scanning schemes forone color, application of the scanning scheme to each color willimplement color printing with plural colors of inks.

The principle of the present invention is applicable not only to thecolor printing but to the monochrome printing. The present invention isalso applicable to the printing that expresses each pixel with aplurality of dots to attain multi-tones. The present invention isfurther applicable to drum scan printers. In the drum scan printer, therotating direction of the drum corresponds to the main scanningdirection, and the feeding direction of the carriage corresponds to thesub-scanning direction. The present invention is applicable not only tothe ink jet printers but in general to any dot recording apparatusesthat record dots on the surface of a printing medium with a recordinghead having plural arrays of dot-forming elements. The “dot-formingelements” here denote elements for forming the dots, such as the inknozzles in the ink jet printer.

The structure embodied by hardware circuitry in the above embodimentscan be replaced by software, and on the contrary, the structure embodiedby software can be replaced by hardware circuitry. For example, thefunction of the control circuit 40 of the color printer 22 (FIG. 2) maybe implemented by the computer. In this case, a computer program such asthe printer driver 96 executed the same control function as that of thecontrol circuit 40.

The computer programs for implementing those functions are provided asstored on a computer readable medium, such as floppy disks or CD-ROMs.The computer 90 reads the computer programs from the storage medium andtransfer them to the internal storage device or to the external storagedevice. Alternatively the computer programs may be supplied from aprogram supply apparatus to the computer 90 via a communications path.At the time of executing the functions of the computer programs, theprograms stored in the main memory are executed by the microprocessor ofthe computer 90. Alternatively, the computer 90 may read out computerprograms stored on the storage medium to directly execute it.

In the specification hereof, the term computer 90 implies both thehardware and its operating system and more specifically represents thehardware operating under the control of the operating system. Thecomputer programs cause the computer 90 to implement the abovefunctions. Part of these functions may be implemented by the operatingsystem instead of the applications programs.

The “computer readable medium” in the present invention is notrestricted to the portable storage medium, but includes a variety ofinternal storage devices in the computer, for example, RAMs and ROMs,and external storage devices connected with the computer, for example,hard disks.

As described above, under the condition of the 3-value outputs, thepresent invention newly spouts an ink droplet upon the dot formed inadvance, thereby forming a nearly complete round dot of a greaterdiameter. This arrangement alleviates the occurrence of banding andensures the high-quality multi-value outputs without requiringcomplicated control.

Moreover, the superposing of the dots having different densities ensuresthe more minute multi-value outputs.

The arrangement of the ink jet printer according to the presentinvention is applicable to any printer that jets ink droplets with avariety of actuators, such as piezoelectric elements and heaters.

What is claimed is:
 1. An ink jet recording apparatus comprising: aprint head having a plurality of nozzles; a main scan driving unit thatdrives the print head in a predetermined main scanning directionrelative to a printing medium; a sub-scan driving unit that drives andfeeds the printing medium in a sub-scanning direction, which isperpendicular to the main scanning direction; a driving unit controllerthat controls the main scan driving unit and the sub-scan driving unitto position the print head at predetermined locations; a data storageunit that stores print image data including multi-value toneinformation; and a print head-driving unit that supplies electric powerto the print head to jet ink onto the printing medium based on the printimage data stored in the data storage unit; the print head including aplurality of nozzle groups, each nozzle group forming dots ofsubstantially identical color, the print head being driven to enableeach nozzle group to record all pixels in an effective recording area onthe printing medium; wherein the print head-driving unit has amulti-value output mode in which the print head is driven so that theprint head can put a plurality of dots having the substantiallyidentical color one upon another at an identical position using theplurality of nozzle groups, to thereby form multi-value dotsrepresenting multi-levels; wherein each of the plurality of nozzlegroups includes N nozzles (N being a positive integer) arranged at anozzle interval k (k being an integer of no less than 2) in thesub-scanning direction; and wherein when the number of used nozzles inthe sub-scanning direction in each nozzle group used for printing isequal to n (n being a positive integer of not greater than N), k and nare prime to each other.
 2. An ink jet recording apparatus comprising: aprint head having a plurality of nozzles; a main scan driving unit thatdrives the print head in a predetermined main scanning directionrelative to a printing medium; a sub-scan driving unit that drives andfeeds the printing medium in a sub-scanning direction, which isperpendicular to the main scanning direction; a driving unit controllerthat controls the main scan driving unit and the sub-scan driving unitto position the print head at predetermined locations; a data storageunit that stores print image data including multi-value toneinformation; and a print head-driving unit that supplies electric powerto the print head to jet ink onto the printing medium based on the printimage data stored in the data storage unit; the print head including aplurality of nozzle groups, each nozzle group forming dots ofsubstantially identical color, the print head being driven to enableeach nozzle group to record all pixels in an effective recording area onthe printing medium; wherein the print head-driving unit has amulti-value output mode in which the print head is driven so that theprint head can put a plurality of dots having the substantiallyidentical color one upon another at an identical position using theplurality of nozzle groups, to thereby form multi-value dotsrepresenting multi-levels; wherein the plurality of nozzle groupsinclude an even nozzle array and an odd nozzle array, each having Nnozzles (N being a positive integer) arranged at a nozzle interval 2k(being an integer of no less than 2) in the sub-scanning direction, andthe even and odd nozzle arrays are apart from each other by apredetermined distance in the main scanning direction; and wherein whenthe number of used nozzles in the sub-scanning direction in each of theeven and odd nozzle arrays used for printing is equal to n (n being apositive integer of not greater than N), 2k and n are prime to eachother.
 3. An ink jet recording apparatus in accordance with claim 1,wherein the print head driving unit puts the plurality of dots havingthe substantially identical color one upon another so that themulti-value dots is substantially circular.
 4. An ink jet recordingapparatus in accordance with claim 1, wherein the plurality of dotshaving the substantially identical color include a first density dothaving a relatively low density and a second density dot having arelatively high density; wherein the multi-levels include a first tonelevel attained by the first density dot, a second tone level attained bythe second density level, and a third tone level attained by superposingthe first and second density dots; and wherein the plurality of nozzlegroups include at least one nozzle group for each of the first andsecond density dots, respectively.
 5. An ink jet recording apparatus inaccordance with claim 1, wherein the plurality of nozzle groups includeat least two nozzle groups for at least one of the first density dot andthe second density dot, the at least two nozzle groups being able torecord all the pixels in the effective recording area; and wherein themulti-levels further include a tone level at which the at least nozzlegroups are used to superpose a plurality of identical density dots oneupon another.
 6. An ink jet recording apparatus in accordance with claim1, wherein the plurality of nozzle groups include at least two nozzlegroups for each of the first density dot and the second density dot, theat least two nozzle groups being able to record all the pixels in theeffective recording area; and wherein the multi-levels further include afourth tone level at which a plurality of the first density dots arelaid one upon another and a fifth tone level at which a plurality of thesecond density dots are laid one upon another.
 7. An ink jet recordingapparatus in accordance with claim 1, wherein the data storage unitincludes a plurality of data blocks for an identical ink, each of theplurality of data blocks storing one bit of pixel information of printimage data; and wherein the plurality of data blocks are related to theplurality of nozzle groups so that 1-bit print image data in each datablock is used as data for the related nozzle group.
 8. An ink jetrecording apparatus in accordance with claim 1, wherein the driving unitcontroller has a medium-feed operation mode in which a feed amount ofthe sub-scan driving unit is fixed to n dots.
 9. An ink jet recordingapparatus in accordance with claim 1, wherein the driving unitcontroller uses a combination of a plurality of different values forfeed amounts of a plurality of sub-scans.
 10. An ink jet recordingapparatus in accordance with claim 1, wherein the print head carries outa plurality of ink-droplet jetting operations for the plurality of dotsof the substantially identical color, the plurality of operations beingcarried out in different main scans, respectively.
 11. A computerreadable recording medium storing a computer program used in a computerthat comprises a printing unit having a plurality of nozzle groups forforming dots of a substantially identical color and a data storage unitfor storing print image data including multi-value tone information, thecomputer program being used for forming dots on a printing medium withthe print head; wherein the recording medium storing the computerprogram for causing the computer to implement a print head drivingfunction for controlling spout of ink droplets on the printing mediumbased on print image data; wherein the print head driving function has amulti-value output mode in which a plurality of dots having thesubstantially identical color are laid one upon another at an identicalposition by the plurality of nozzle groups, to thereby form multi-valuedots representing multi-levels; wherein each of the plurality of nozzlegroups includes N nozzles (N being a positive integer) arranged at anozzle interval k (k being an integer of no less than 2) in thesub-scanning direction; and wherein when the number of used nozzles inthe sub-scanning direction in each nozzle group used for printing isequal to n (n being a positive integer of not greater than N), k and nare prime to each other.
 12. A computer readable recording mediumstoring a computer program used in a computer that comprises a printingunit having a plurality of nozzle groups for forming dots of asubstantially identical color and a data storage unit for storing printimage data including multi-value tone information, the computer programbeing used for forming dots on a printing medium with the print head;wherein the recording medium storing the computer program for causingthe computer to implement a print head driving function for controllingspout of ink droplets on the printing medium based on print image data;wherein the print head driving function has a multi-value output mode inwhich a plurality of dots having the substantially identical color arelaid one upon another at an identical position by the plurality ofnozzle groups, to thereby form multi-value dots representingmulti-levels; wherein the plurality of nozzle groups include an evennozzle array and an odd nozzle array, each having N nozzles (N being apositive integer) arranged at a nozzle interval 2k (k being an integerof no less than 2) in the sub-scanning direction, and the even and oddnozzle arrays are apart from each other by a predetermined distance inthe main scanning direction; and wherein when the number of used nozzlesin the sub-scanning direction in each of the even and odd nozzle arraysused for printing is equal to n (n being a positive integer of notgreater than N), 2k and n are prime to each other.
 13. An ink jetrecording apparatus in accordance with claim 12, wherein the print headdriving unit puts the plurality of dots having the substantiallyidentical color one upon another so that the multi-value dots issubstantially circular.
 14. An inkjet recording apparatus in accordancewith claim 12, wherein the plurality of dots having the substantiallyidentical color include a first density dot having a relatively lowdensity and a second density dot having a relatively high density;wherein the multi-levels include a first tone level attained by thefirst density dot, a second tone level attained by the second densitylevel, and a third tone level attained by superposing the first andsecond density dots; and wherein the plurality of nozzle groups includeat least one nozzle group for each of the first and second density dots,respectively.
 15. An ink jet recording apparatus in accordance withclaim 12, wherein the plurality of nozzle groups include at least twonozzle groups for at least one of the first density dot and the seconddensity dot, the at least two nozzle groups being able to record all thepixels in the effective recording area; and wherein the multi-levelsfurther include a tone level at which the at least nozzle groups areused to superpose a plurality of identical density dots one uponanother.
 16. An ink jet recording apparatus in accordance with claim 12,wherein the plurality of nozzle groups include at least two nozzlegroups for each of the first density dot and the second density dot, theat least two nozzle groups being able to record all the pixels in theeffective recording area; and wherein the multi-levels further include afourth tone level at which a plurality of the first density dots arelaid one upon another and a fifth tone level at which a plurality of thesecond density dots are laid one upon another.
 17. An ink jet recordingapparatus in accordance with claim 12, wherein the data storage unitincludes a plurality of data blocks for an identical ink, each of theplurality of data blocks storing one bit of pixel information of printimage data; and wherein the plurality of data blocks are related to theplurality of nozzle groups so that 1-bit print image data in each datablock is used as data for the related nozzle group.
 18. An ink jetrecording apparatus in accordance with claim 12, wherein the drivingunit controller has a medium-feed operation mode in which a feed amountof the sub-scan driving unit is fixed to n dots.
 19. An ink jetrecording apparatus in accordance with claim 12, wherein the drivingunit controller uses a combination of a plurality of different valuesfor feed amounts of a plurality of sub-scans.
 20. An ink jet recordingapparatus in accordance with claim 12, wherein the print head carriesout a plurality of ink-droplet jetting operations for the plurality ofdots of the substantially identical color, the plurality of operationsbeing carried out in different main scans, respectively.